Nano-material dispersion 101: a brief intro

Most practical uses of nano-materials require their dispersion in various solvents, for example, to enable embedding the nano-materials in non-aggregated form in a product. However, such dispersions are intrinsically thermodynamically unstable due to increased surface energy. What it means in plain language is that the particles don’t like being separated and would strongly tend to aggregate spontaneously. It also means that, sooner or later, they will get their wish, or, technically speaking, every dispersion has certain finite lifetime. Finally, it means a lot of external work is required to disperse nano-materials and then prevent them from re-aggregating.

Generally, the smaller the nanoparticles, the less stable the dispersed state would be, and more work would be needed to achieve smaller particle size. In the case of carbon nanotubes (CNT), the ultimate dispersion would entail having only individual CNTs. In reality, all dispersions will comprise bundles of CNTs together with individual CNTs in various proportions. With increasing storage time, the bundle size inevitably tends to increase, and the time frame of this increase would depend on particular dispersion.

Another important factor to have in mind is the concentration of CNT in a solvent. At low concentration levels, such as 0.01wt% or less, a good dispersion may contain predominantly individual CNTs interspaced with small CNT bundles in various proportions. However, at higher CNT concentrations, such as 0.1wt% or above, the dominant species in the dispersion would be CNT bundles.

Working with currently existing CNT dispersions requires a lot of advanced skills and knowledge if a company wants to explore using CNT materials in their products. Without this specialized expertise, it would be difficult to make proper use of the existing CNT dispersions. For example, removal of surfactants or elimination of bulky functional groups from final product are not straightforward processes. They depend on a number of conditions and are more form of art rather than well-defined universal technical procedure. 

Our unique CNT dispersions which are free from all foreign additives remove the veil of mystery from CNTs and make them available for use by everyone, even those without special skills or training. With “drop-in” technologies enabled by the Voxelum dispersions, working with CNTs becomes simple and easy. If your company needs to incorporate CNTs into your products and unleash the full potential of CNT materials, you don’t have to hire a full-time CNT expert. All you have to do is buy a CNT dispersion from Voxelum and follow simple straightforward instructions. And, if you still have any questions, please remember that your friends at Voxelum are available to answer them and help you succeed with your product.

 

Why it’s necessary to disperse CNTs

CNTs are known to have many unique properties, such as exceptionally high electrical and thermal conductivity, high tensile strength and Young modulus, high thermal and chemical stability, decreased flammability, etc., potentially leading to many commercially attractive applications. For example, addition of even small amounts of CNT to polymer, ceramic, or other materials is expected to confer the ability to dissipate electrostatic charges, as well as bring about notable improvements in properties. However, these improvements can't be practically utilized using CNT powders - which is how most CNTs are offered commercially. In a powder form, CNTs are heavily aggregated resulting in loss or weakening of their useful properties. In most applications, to fully realize their tremendous potential, CNT materials have to be in a well-dispersed form. 

 

Existing CNT dispersions:
what is good and what is bad about them

Currently, there are two main approaches to CNT dispersions, both relying on external agents (additives) to keep CNTs from re-aggregating. Presence of these additives may weaken the positive effect of CNTs and/or require extensive additional processing for their removalHence, neither of these approaches represents a “drop-in” type solution, and absence of such convenient solutions is impeding large scale commercial utilization of CNT-based products.

First approach is based on breaking CNT aggregates via ultrasonic or high shear treatment and using large amounts of dispersing agents, such as surfactants, to prevent CNT re-aggregation. The surfactant-based method is not feasible for a number of applications, for example, in media that are not tolerant to that type of impurities. Bulky surfactant molecules envelope individual CNTs and CNT bundles thereby sharply reducing the fraction of exposed CNT surface and impacting product properties. This approach necessitates complex and material-dependent post-processing steps to remove surfactants leading to increased cost, time, and complexity, as well as generating notable amounts of waste. In the process, some CNTs are also lost making it difficult to evaluate the precise amounts of both residual surfactant and CNTs in the product. Presence of even small, residual surfactant quantities may still negatively impact product performance, for example, electrical conductivity (see e.g., J. Wang et al., Removal of the Residual Surfactants in Transparent and Conductive Single-Walled Carbon Nanotube Films). Finally, removal of surfactants often requires multiple washes with copious amounts of water. This process may generate waste water in volumes comparable to or larger than the product itself thereby sharply increasing the use of pure water resource beyond the volumes needed as solvent. Additionally, contamination of the waste water with surfactants and especially CNTs poses problems with waste disposal and environmental mitigation

Second approach involves functionalizing CNTs, that is, deliberately attaching special functional groups to CNTs. Strictly speaking, all CNTs have some "naturally' occurring functional groups, but these normally don't make them soluble. To impart solubility, functional groups should be of specific type and their concentration should be orders of magnitude higher compared to the "naturally' occurring ones. For example, polyethyleneglycol (PEG) and octadecylamine (ODA) groups impart good solubility in water and organic solvents, respectively. Properly functionalized CNTs readily form stable dispersions by ultrasonic treatment. The main problem with this approach is that functionalizing CNTs is an expensive and destructive process which may generate large amounts of hazardous waste. The process is also difficult to scale up making it impractical for most commercial applications. Functionalized CNTs may have impaired properties such as chemical and thermal stability, tensile strength, electrical conductivity due to induced defects, sterical hindering by bulky functional groups, and disruption of the CNT backbone conjugated structure (see e.g., J. Hilding et al., Dispersion of Carbon Nanotubes in Liquids). Use of functionalized CNTs may not be feasible in some applications, such as in electrochemistry, due to increased chemical reactivity of the functional groups. The attached functional groups can be removed by post-processing step of annealing the product at high temperatures under inert atmosphere, thereby adding cost, time, and complexity. Such annealing step may not be feasible for products which are sensitive to high temperatures.

 

How to select additive-free CNT dispersion
that is right for you

To select a CNT dispersion from our menu, please specify the following set of parameters: 

  • (1) type of CNT (either single-wall, SWCNT, or multi-wall, MWCNT)
  • (2) type of solvent (either water or isopropyl alcohol, IPA)
  • (3) CNT concentration in the solvent
  • (4) amount (volume) of the dispersion

Selecting the CNT dispersion parameters depends entirely on your particular application. If you are unsure which CNT dispersion would work best for you, please contact us – we can help. For some applications, it is important to have specific CNT type (i.e., single-wall or multi-wall), solvent (water, IPA, or other), purity level, range of diameter distribution and aspect ratio of the CNT material. Some applications are more price-sensitive than others, and you may be interested to know whether the CNT dispersions may be available at lower price levels for large scale use. If any of the above represent your case, please contact us to discuss custom making the right additive-free CNT dispersion for you.

Here are a few tips on how to decide which type of carbon nanotubes is best for you. First, what is the main difference between SWCNT and MWCNT from the perspective of applications? These two sets of CNT materials have some properties that are largely interchangeable, but they also have fundamental differences. For example, both SWCNT and MWCNT can impart electrical conductivity and mechanical strength to various polymer matrices. However, composites based on SWCNT are typically more conductive at the same level of loading and other parameters. If your application requires presence of a semiconducting band gap, then your only choice is SWCNT - since MWCNTs don't have semiconducting properties. Also, SWCNT materials typically have higher surface area compared to MWCNTs, so they are preferable for applications where high surface area is important. 

The photograph below shows four CNT dispersions in plastic boats. The quality of dispersions can be evaluated visually from these photographs. The left bottom boat shows a non-uniform, poor quality dispersion comprised of aggregated CNT particles. Phase separation into CNT and solvent can be clearly seen indicating unstable dispersion.

The other three photos show good quality dispersions with CNT particles uniformly distributed in solvents. No phase separation is observed. The upper right photo shows typical well-dispersed, uniform, good quality dispersion containing multi-wall CNT in IPA solvent at low concentration (about 0.001wt%). The dispersions in the upper left (IPA solvent) and lower right (water solvent) appear much darker due to the higher multi-wall CNT concentration (about 0.1wt%).

Unique Voxelum technology for additive-free CNT dispersion

Voxelum has developed a unique technology to make stable CNT dispersions without using any surfactants or covalent functionalization. Voxelum dispersions comprise only pure, chemically unmodified CNTs (as received from manufacturers) and common, safe solvents such as water or isopropyl alcohol. The Voxelum technology is scalable and works with most CNT materials, both single-wall (SWCNT) and multi-wall (MWCNT), as well as with a number of solvents. We believe that our CNT dispersions are potentially game-changing by representing the first “drop-in” technology for introducing CNTs into various matrices and systems with minimum effort and to maximum effect.

Voxelum technology is the most advanced in terms of sustainability and environmental mitigation and fully scalable. Emergence of such technologies will greatly facilitate commercially viable applications of CNT materials in various industries.

We provide CNT dispersions using our own stock of high quality CNTs sourced from leading reputable CNT manufacturers. Alternatively, you can send us CNT material of your choosing, and we will make a custom dispersion based on your own material.

 

Try them out!

·         Will your application work better if you use Voxelum dispersions? Will your product acquire higher tensile strength and/or electrical or thermal conductivity? Will you also get better control over your process, save cost, and eliminate extra steps and waste?

·         To find out, you should buy a trial amount sample (25ml size) and test it. It doesn't cost much and it may revolutionize your product - or just make it better and less expensive.

·         Voxelum dispersions are normally stable over several months of storage at temperatures 10ºC to 60ºC for aqueous dispersions, or -30ºC to 40ºC for IPA dispersions. Exposure to extreme temperatures or strong vibrations should be avoided. 

·         Please note that Voxelum dispersions do not always behave similar to surfactant-based or functionalized CNT-based dispersions. To refresh your Voxelum dispersion after extended time (4-6 months or longer) storage, you can stir it manually, for example, with a spatula, or using a magnetic stirrer. We do not recommend using any type of ultrasonic treatment. We also recommend that Voxelum dispersions be used as received without diluting since it may lead to irreversible degradation of the dispersion.

  • If you purchased our dispersion and have any questions about how best to use it, or you need a more customized product, please feel free to contact us – we are happy to help you succeed. Please note that the minimum volume for customized dispersion orders is 25ml